1. Field of the Invention
The present invention relates to display apparatuses including light-emitting devices and particularly relates to a display apparatus including organic electroluminescent (EL) diodes which emit light upon application of electric fields to thin films containing an organic compound.
2. Description of the Related Art
Organic EL diodes include anodes, cathodes, and thin films which contain fluorescent or phosphorescent organic compounds and which are sandwiched between the anodes and cathodes. The organic EL diodes emit light when electrons and holes are injected into the thin films from the cathodes and the anodes, respectively, and when the electrons are combined with the holes to generate excitons from the fluorescent or phosphorescent organic compound. The excitons transit from an excited state to a ground state and light is emitted. A study conducted by Eastman Kodak Company in 1987 reports on a diode having two layers with different functions in Appl. Phys. Lett., 51, p. 913, (1987). The diode has a brightness of about 1,000 cd/m2 at an applied voltage of about 10 V. The diode includes an anode made of indium tin oxide (ITO) and a cathode made of a silver-magnesium (Ag—Mg) alloy and contains an aluminum-quinolinol complex having an electron-transporting ability and a light-emitting ability and a triphenylamine derivative having a hole-transporting ability. Patents relating to the diode include U.S. Pat. Nos. 4,539,507, 4,720,432, and 4,885,211.
FIG. 1A shows an ordinary organic EL diode, which includes a reflective electrode 102, a transparent electrode 103 located on the side from which light is extracted, and an organic layer 101 sandwiched therebetween. A first light beam 202 emitted from a light-emitting point 201 is transmitted to the transparent electrode 103 and a second light beam 203 emitted therefrom is reflected by the reflective electrode 102 and then transmitted toward the transparent electrode 103. Hence, the amount of light extracted through the transparent electrode 103 is large, that is, the organic EL diode has increased light-emitting efficiency.
An external light beam 301 incident on the organic EL diode is reflected by the reflective electrode 102 as shown in FIG. 1B. Therefore, there is a problem in that a display including the organic EL diode has low contrast and visibility. This problem is not peculiar to organic EL displays but is common to light-emitting displays, such as quantum-dot light-emitting diode (QD-LED) displays, inorganic EL displays, and plasma displays, including reflective electrodes.
In order to improve low contrast images, a technique is employed in which right- or left-circularly polarized light is converted into left- or right-circularly polarized light, respectively, by reflection. Japanese Patent No. 2761453 and Japanese Patent Laid-Open No. 2005-332815 each discloses a technique in which the reflection of external light is prevented by placing a circularly polarizing plate on the side from which light is extracted.
Alternatively, to improve contrast and prevent reflection, interference is generated in a stack of thin films having a thickness of several ten to several hundred micrometers. Japanese Patent No. 3555759 discloses a technique in which interference occurring in an optical resonator is used in combination with a monochromatic absorption filter and a technique in which interference occurring in an optical resonator is used in combination with red, green, and blue color filters each corresponding to a pixel.
Although the circularly polarizing plate used in the technique disclosed in Japanese Patent No. 2761453 has a simple structure and is superior in preventing the reflection of light, ordinary circularly polarizing plates have a transmittance of 35% to 45% in the visible range (the theoretical upper limit of the transmittance is 50%). Therefore, there is a problem in that the use of such circularly polarizing plates reduces by half the light-emitting efficiency of organic EL diodes independently of display colors. In order to solve this problem, Japanese Patent Laid-Open No. 2005-332815 discloses that the spectral transmittance of a circularly polarizing plate and/or the degree of spectral polarization thereof is varied depending on the light-emitting efficiency of an organic EL diode.
However, there is a problem in that if light-emitting efficiency is increased by increasing the spectral transmittance of a circularly polarizing member, the ability of preventing the reflection of external light is deteriorated because an increase in spectral transmittance reduces the degree of spectral polarization and the polarizing ability of the circularly polarizing member.